Problem: Jessica is 3 times as old as Stephanie. Eighteen years ago, Jessica was 9 times as old as Stephanie. How old is Jessica now?
We can use the given information to write down two equations that describe the ages of Jessica and Stephanie. Let Jessica's current age be $j$ and Stephanie's current age be $s$ The information in the first sentence can be expressed in the following equation: $j = 3s$ Eighteen years ago, Jessica was $j - 18$ years old, and Stephanie was $s - 18$ years old. The information in the second sentence can be expressed in the following equation: $j - 18 = 9(s - 18)$ Now we have two independent equations, and we can solve for our two unknowns. Because we are looking for $j$ , it might be easiest to solve our first equation for $s$ and substitute it into our second equation. Solving our first equation for $s$ , we get: $s = j / 3$ . Substituting this into our second equation, we get: $j - 18 = 9($ $(j / 3)$ $- 18)$ which combines the information about $j$ from both of our original equations. Simplifying the right side of this equation, we get: $j - 18 = 3 j - 162$ Solving for $j$ , we get: $2 j = 144$ $j = \dfrac{1}{2} \cdot 144 = 72$.